跳过-with-块的执行

Question

我正在定义一个上下文管理器类，并且如果实例化过程中满足某些条件，我希望能够跳过代码块而不会引发异常。例如，

class My_Context(object):
    def __init__(self,mode=0):
        """
        if mode = 0, proceed as normal
        if mode = 1, do not execute block
        """
        self.mode=mode
    def __enter__(self):
        if self.mode==1:
            print 'Exiting...'
            CODE TO EXIT PREMATURELY
    def __exit__(self, type, value, traceback):
        print 'Exiting...'

with My_Context(mode=1):
    print 'Executing block of codes...'

Answer 1

如果您想要一个临时解决方案，它使用withhacks（特别是来自AnonymousBlocksInPython）的思想，那么它将起作用：

import sys
import inspect

class My_Context(object):
    def __init__(self,mode=0):
        """
        if mode = 0, proceed as normal
        if mode = 1, do not execute block
        """
        self.mode=mode
    def __enter__(self):
        if self.mode==1:
            print 'Met block-skipping criterion ...'
            # Do some magic
            sys.settrace(lambda *args, **keys: None)
            frame = inspect.currentframe(1)
            frame.f_trace = self.trace
    def trace(self, frame, event, arg):
        raise
    def __exit__(self, type, value, traceback):
        print 'Exiting context ...'
        return True

比较以下内容：

with My_Context(mode=1):
    print 'Executing block of code ...'

与

with My_Context(mode=0):
    print 'Executing block of code ... '